Procedures for determining the presence or absence of specific organisms or viruses in a test sample commonly rely upon nucleic acid-based probe testing. To increase the sensitivity of these tests, an amplification step is often included to increase the number of potential nucleic acid target sequences present in the test sample. There are many procedures for amplifying nucleic acids which are well known in the art, including, but not limited to, the polymerase chain reaction (PCR), (see, e.g., Mullis, “Process for Amplifying, Detecting, and/or Cloning Nucleic Acid Sequences,” U.S. Pat. No. 4,683,195), transcription-mediated amplification (TMA), (see, e.g., Kacian et al., “Nucleic Acid Sequence Amplification Methods,” U.S. Pat. No. 5,399,491), ligase chain reaction (LCR), (see, e.g., Birkenmeyer, “Amplification of Target Nucleic Acids Using Gap Filling Ligase Chain Reaction,” U.S. Pat. No. 5,427,930), and strand displacement amplification (SDA), (see, e.g., Walker, “Strand Displacement Amplification,” U.S. Pat. No. 5,455,166). A review of several amplification procedures currently in use, including PCR and TMA, is provided in HELEN H. LEE ET AL., NUCLEIC ACID AMPLIFICATION TECHNOLOGIES (1997).
A concern with amplification is the possibility of cross-contamination, since transferring even a minute amount of target-containing sample to a target-negative sample could lead to the production of billions of target sequences in the “negative” sample. As a consequence, a test may indicate a positive result for a sample actually lacking nucleic acid from an organism or virus of interest. The source of a contaminating sample transfer may be an aerosol or bubbles released from a sample tube when a cap component of the sample tube is removed by a practitioner or instrument. To minimize such sources of contamination, penetrable caps having filtering means were recently introduced and are disclosed by Anderson et al., “Collection Device and Method for Removing a Fluid Substance from the Same,” U.S. Patent Application No. 20010041336 A1, and Kacian et al., “Penetrable Cap,” U.S. application Ser. No. 10/093,511, both of which enjoy common ownership herewith.
To limit the force required to penetrate a sample tube having a penetrable cap, it is important for the penetrable surface of the cap to be centered under a robotic pipettor in an automated sampling system. See, e.g., Ammann et al., “Automated Process for Isolating and Amplifying a Target Nucleic Acid Sequence,” U.S. Pat. No. 6,335,166, which enjoys common ownership herewith (an instrument for performing amplification assays on test samples which includes a robotic pipettor for obtaining test sample from a sample tube is disclosed). By centering the penetrable cap, a pipette tip fixed to the robotic pipettor may be programmed to contact and pierce a weak point on the cap. See, e.g., Anderson et al., U.S. patent application Ser. No. 20010041336 A1 (a plastic, conically-shaped, striated cap is disclosed in one embodiment). And, if the filtering means included in the penetrable cap provides the least resistance if it is centered under the cap, as with a material which is rolled or contains a center cut or bore, then the pipette tip will encounter the least resistance with the filter if the pipette tip is centered on the penetrable surface of the cap.
Conventional sample carriers commonly rely upon springs to immobilize distal ends of sample tubes, biasing the sample tubes against one or more opposing surfaces of the sample carriers. While these sample carriers are generally adequate to hold open-ended sample tubes during transport and pipetting in an automated sampling system, they do not include a mechanism for maintaining the sample tubes in fixed vertical orientations or for centering sample tubes having vessel components of varying diameters. As a result, conventional sample carriers are unreliable for holding and centering sample tubes having penetrable caps whose design and construction requires accurate positioning of the sample tubes in order to minimize the forces needed to penetrate the caps with a robotic pipettor.
Thus, a need exists for a sample carrier which maintains sample tubes in fixed vertical orientations, permitting penetrable cap components of the sample tubes to be centered under and pierced by a robotic pipettor within an automated sampling system using minimal force. By centering closed sample tubes for penetration by a robotic pipettor, filters contained within the caps of the sample tubes should be able function optimally as barriers to contaminating aerosols and bubbles present in the sample tubes and to remove sample residue from the outer surfaces of pipette tips as they are being withdrawn from the sample tubes.